Alzheimer's disease is a degenerative disorder of the human brain. Clinically, it appears as a progressive dementia. Its histopathology is characterized by degeneration of neurons, gliosis, and the abnormal deposition of proteins in the brain. Pathological hallmarks include neurofibrillary tangles (paired helical filaments) and amyloid deposits within the parenchyma and cerebral vasculature.
While there is no general agreement as to the chemical nature of neurofibrillary tangles, the major constituent of both the amyloid plaque cores and the amyloid of the congophilic angiopathy has been shown to be a 4500 Dalton protein originally termed .beta.-protein or amyloid A4. Throughout this document this protein is referred to as .beta.-amyloid peptide or protein.
.beta.-amyloid peptide is proteolytically derived from a transmembrane protein, the amyloid precursor protein. Different splice forms of the amyloid precursor protein are encoded by a widely expressed gene. see, e.g., K. Beyreuther and B. Muller-Hill, Annual Reviews in Biochemistry, 58:287-307 (1989). .beta.-amyloid peptide consists, in its longest forms, of 42 or 43 amino acid residues. J. Kang, et al., Nature (London), 325:733-736 (1987). These peptides, however, vary as to their amino-termini. C. Hilbich, et al., Journal of Molecular Biology, 18:149-163 (1991).
Because senile plaques are invariably surrounded by dystrophic neurites, it was proposed early that .beta.-amyloid peptide is involved in the loss of neuronal cells that occurs in Alzheimer's disease. B. Yankner and co-workers were the first to demonstrate that synthetic .beta.-amyloid peptide could be neurotoxic in vitro and in vivo. B. A. Yankner, et al., Science, 245:417 (1989; See, also, N. W. Kowall, et al., Proceedings of the National Academy of Sciences, U.S.A., 88:7247 (1991). Other research groups, however, were unable to consistently demonstrate direct toxicity with .beta.-amyloid peptide. See, e.g., Neurobiology of Aging, 13:535 (K. Kosik and P. Coleman, eds. 1992). Even groups receiving .beta.-amyloid peptide from a common source demonstrate conflicting results. D. Price, et al., Neurobiology of Aging, 13:623-625 (1991)(and the references cited therein).
Because of the debilitating effects of Alzheimer's disease there continues to exist a need for effective treatments. This invention provides methods for the treatment of Alzheimer's disease in mammals.
Recent studies have begun to indicate that a major component of the pathology of Alzheimer's disease is chronic inflammation. See, J. Schnabel, Science, 260:1719-1720 (1993). Indeed, pathological investigations have demonstrated the presence of glial hyperactivity, acute phase proteins, and complement factors within affected areas of the brains of persons affected with Alzheimer's disease. Administration of nonsteroidal anti-inflammatory drugs appears to slow the advance of Alzheimer's disease. Id. Understanding this inflammatory component of Alzheimer's disease, therefore, will lead to advances in novel methods of treating patients suffering from this disease.
Inflammatory disorders account for a significant number of debilitating diseases. Inflammatory states, such as arthritis, psoriasis, asthma, and possibly atherosclerosis, stem from inflammatory reactions in the joints, skin, and blood vessels. It is generally believed that a central role in the inflammatory reaction is the production of phospholipid metabolites called eicosanoids. The eicosanoids represent a family of important mediators such as the leukotrienes, prostaglandins, lipoxins, hydroxyeicosatetranoic acid, and thromboxanes. It is believed that the generation of eicosanoids is dependent on the availability of arachidonic acid which is liberated from phospholipids by the action of phospholipase A.sub.2 (EC 3.1.1.4).
Phospholipase A.sub.2 (PLA.sub.2) is the common name for phosphatide 2-acylhydrolase, which catalyzes the hydrolysis of the sn-2-acyl ester bond of phosphoglycerides which results in the production of equimolar amounts of lysophospholipids and free fatty acids. see, E. A. Dennis, "The Enzymes", Vol. 16, Academic Press, New York, (1983). Phospholipase A.sub.2 enzymes are found in all living species and form a diverse family of enzymes. Over forty phospholipase A.sub.2 enzymes have been structurally characterized, and they show a high degree of sequence homology. J. Chang, et al., Biochemical Pharmacology, 36:2429-2436, (1987).
The best characterized varieties of PLA.sub.2 enzyme are the secreted forms, which are released into the extracellular environment where they aid in the digestion of biological materials. The secreted forms have a molecular weight of about 12-15,000 (Chang, et al, supra). In contrast, cytosolic phospholipases A.sub.2 are found in small amounts within the cell and play a key role in the biosynthetic pathway leading to the formation of the platelet activating factors and the eicosanoids. D. Mobilio and L. A. Marshall, Annual Reports in Medicinal Chemistry, 24; 157-166, (1989).
The cytosolic phospholipases A.sub.2 have a molecular weight of approximately 85,000 daltons. J. D. Clark, et al., Cell, 65:1043-1051 (1991). Free arachidonic acid is the rate limiting precursor for the production of eicosanoids and is liberated from its membrane phospholipid store by the action of cytosolic PLA.sub.2. E. A. Dennis, Drug Development and Research, 10:205-220, (1987). The same enzymatic step also produces lysophospholipids which may be converted to platelet-activating factors. Thus, it is believed that cytosolic PLA.sub.2 is central to the regulation of the biosynthetic pathways of potent lipid mediators of inflammation.
Due to the central role in the inflammatory component of Alzheimer's disease that appears to be played by cytosolic phospholipase A.sub.2, it is desirable to identify and characterize new inhibitors of this enzyme.